Pyrochemical processes are known to produce uranium (U), thorium (Th) and plutonium (Pu) metals by the reduction of their respective oxides using magnesium in the presence of a molten salt flux. Often in such processes, a separate molten metal pool is employed for the recovery of the actinide metal from the molten salt phase. The flux is used to suspend the actinide metal oxide and the byproduct magnesium oxide during the reaction, and the heavier metallic pool is used to extract the metal produced. When I refer to the term "suspend" in the context of these processes, I mean that the molten salt flux may either dissolve the actinide metal oxide and/or the magnesium oxide or it may suspend the oxide as fine particles.
The chemistry of such pyrochemical reduction reactions is well known. At a suitable temperature, e.g., 700.degree. C., the actinide metal oxide reacts with a chemically equivalent amount of magnesium to form the respective actinide metal and magnesium oxide. For example, uranium dioxide (UO.sub.2) reacts with two moles of magnesium to form two moles of magnesium oxide plus the product, one mole of substantially pure uranium. As has been practiced, a suitable salt flux for this magnesium reduction practice may, for example, consist of magnesium chloride and magnesium fluoride with the optional presence of other alkali and alkaline earth chlorides or fluorides. The molten metallic recovery pool typically initially consists essentially of zinc and magnesium or copper and magnesium. This metallic pool is present at the reaction temperature as a separate and separable liquid phase with respect to the salt flux reaction medium. The pool contains magnesium in suitable quantity for the chemical reduction of the uranium, thorium and/or plutonium oxide. As the magnesium is consumed in the reduction of e.g. UO.sub.2 the byproduct uranium metal enters the pool. Zinc or copper is miscible with both uranium and magnesium and keeps the metal phase liquid and mobile at the reaction temperature.
These practices are suitable for obtaining relatively pure amounts of the respective actinide metal or metals in high yield. The reactions are usually batch reactions. They are carried out until the actinide oxide(s) are completely reduced. At this point, the reaction stops and stirring is terminated, and a phase separation is carried out to remove the metal recovery pool from the flux. The actinide metal is recovered by distilling or otherwise removing the zinc or copper melting point depressant.
In this reduction practice, magnesium oxide particles are suspended in but not dissolved in the salt flux. This oxide/salt mixture is often radioactive. It presents a disposal problem because it is difficult to recover or recycle the magnesium oxide. In order to recover magnesium metal from the magnesium oxide, it is usually necessary to separate the oxide from the salt and convert the magnesium oxide to magnesium chloride for electrolytic reduction. Heretofore, such separation, conversion and reduction have been difficult and expensive.
It is an object of this invention to provide a practice and a salt flux composition that readily facilitates the conversion of magnesium oxide produced in such pyrochemical reductions of actinide oxides so that the recycle of the salts and recovery and recycle of magnesium metal is readily accomplished. While my practice is specifically applicable to the recovery of magnesium from magnesium oxide byproduct in the pyrochemical reduction of actinide metal oxides, it will be appreciated that there are other applications in which it is desired to convert magnesium oxide to a magnesium salt or to recover magnesium from magnesium oxide. In fact, a principal application may well be the production of magnesium metal from magnesium oxide. In accordance with a preferred embodiment of my invention, these and other objects are accomplished as follows.